In order to improve performance of various safety devices such as a seatbelt and an air bag, operations of such safety devices may be controlled depending on a weight of an occupant seated on a vehicle seat. As a means for measuring the weight of the occupant, i.e., a load thereof, it is known to use a sensor in which a strain gauge is utilized to detect a small deformation of a seat frame, and the like. The strain gauge detects a small change in the shape as a change in electrical resistance to thereby output a weak electrical signal. An amplifier substrate for amplifying the electrical signal is generally used in combination with the strain gauge. In a vehicle interior, however, various electromagnetic noises such as an ignition noise of an engine are emitted from a driving device, a lighting device, and the like. Accordingly, a shield structure for reducing an influence of the electromagnetic noise to the sensor is applied. The shield structure includes a metallic shield member having a thickness in response to a desired shielding performance so as to cover an electronic circuit such as the amplifier substrate to thereby block propagation of the electromagnetic noise.
A noise shield structure for a load detection sensor for a vehicle seat is disclosed in JP2008-134226A (which will be hereinafter referred to as Reference 1). The shield structure disclosed in Reference 1 includes a strain generating body of which both end portions are fixed to one of a floor side mounting member and a seat side mounting member and of which a center portion is fixed to the other one of the floor-side mounting member and the seat side mounting member. The noise shield structure further includes a strain gauge attached to the strain generating body, an amplifier substrate amplifying an output from the strain gauge, and a metallic bracket by means of which the amplifier substrate is arranged next to the strain generating body in parallel thereto. One surface of the amplifier substrate is covered by the bracket while the other surface is covered by a ground pattern made of metallic foil.
According to the shield structure disclosed in Reference 1, because both of the surfaces of the amplifier substrate are covered by the bracket and the ground pattern, the influence of the electromagnetic noise is reduced. In addition, a distance between the strain gauge and the amplifier substrate is extremely short, which leads to a space reduction. Further, because the bracket for attaching the amplifier substrate and the ground pattern of the electronic circuit within the amplifier substrate are used as the shield structure, an additional member for shielding is not required, which leads to a decrease of the number of components and a cost.
The shielding performance against the electromagnetic noise is achieved not only by a specific member but also by operations of all metallic members arranged around the amplifier substrate. According to the shield structure disclosed in Reference 1, not only the bracket and the ground pattern but also the floor side mounting member and the seat side mounting member both of which are made of metal contribute to the shielding performance. Thus, in a case where a shape or an arrangement of the mounting member is changed, the shielding performance is naturally changed. For example, in a case where a positional height of the strain generating body increases by means of a bracket because of a structure of the seat frame, a rail for moving the seat, and the like, a space below the amplifier substrate increases, which leads to an easy intrusion of the electromagnetic noise. The metallic foil of the grand pattern has an insufficient thickness for the adequate shielding performance. As a result, the electromagnetic noise may influence the load detection performance.
A need thus exists for a load detection device for a vehicle seat which is not susceptible to the drawback mentioned above.